Multiple IP Addresses¶

Overview¶

Servers often need multiple IP addresses for:

Virtual hosting (multiple websites)
Service separation (different IPs for different services)
High availability (floating IPs)
Network migration (temporary dual addressing)

Multiple Addresses on One Interface¶

Basic Multiple Addresses¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24    # Primary
        - 192.168.1.101/24    # Secondary
        - 192.168.1.102/24    # Tertiary
      routes:
        - to: default
          via: 192.168.1.1

With Labels (Compatibility)¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24:
            label: "eth0"
        - 192.168.1.101/24:
            label: "eth0:web"
        - 192.168.1.102/24:
            label: "eth0:mail"

Labels help identify addresses in ip addr output and some legacy applications.

Different Subnets¶

Multiple Networks on One Interface¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24    # Primary network
        - 10.0.0.100/24       # Secondary network
        - 172.16.0.100/24     # Third network
      routes:
        - to: default
          via: 192.168.1.1
        - to: 10.0.0.0/8
          via: 10.0.0.1
        - to: 172.16.0.0/12
          via: 172.16.0.1

Public and Private¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 203.0.113.100/24    # Public IP
        - 10.0.0.100/24       # Private IP (management)
      routes:
        - to: default
          via: 203.0.113.1
        - to: 10.0.0.0/8
          via: 10.0.0.1

IPv4 and IPv6¶

Dual Stack with Multiple Addresses¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        # IPv4 addresses
        - 192.168.1.100/24
        - 192.168.1.101/24
        # IPv6 addresses
        - "2001:db8::100/64"
        - "2001:db8::101/64"
      routes:
        - to: default
          via: 192.168.1.1
        - to: "::/0"
          via: "2001:db8::1"

Address Lifetime¶

Permanent Addresses (Default)¶

addresses:
  - 192.168.1.100/24

Addresses with Lifetime¶

addresses:
  - 192.168.1.100/24:
      lifetime: 0           # Permanent (default)
  - 192.168.1.200/24:
      lifetime: 3600        # Expires in 1 hour

Floating/Virtual IPs¶

For High Availability¶

Primary server:

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24    # Real IP
        - 192.168.1.200/24    # Floating IP (when primary)
      routes:
        - to: default
          via: 192.168.1.1

Secondary server (floating IP managed by HA software):

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.101/24    # Real IP
        # Floating IP (192.168.1.200) added by keepalived/pacemaker
      routes:
        - to: default
          via: 192.168.1.1

CIDR Considerations¶

Same Subnet¶

addresses:
  - 192.168.1.100/24
  - 192.168.1.101/24    # Same /24, redundant route

Host Addresses¶

For IPs that shouldn't create subnet routes:

addresses:
  - 192.168.1.100/24    # Creates route for 192.168.1.0/24
  - 10.10.10.10/32      # No route created (loopback-style)

Service Binding Examples¶

Web Server Virtual Hosts¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24    # Main site
        - 192.168.1.101/24    # Blog
        - 192.168.1.102/24    # API

Apache configuration:

<VirtualHost 192.168.1.100:443>
    ServerName www.example.com
</VirtualHost>

<VirtualHost 192.168.1.101:443>
    ServerName blog.example.com
</VirtualHost>

<VirtualHost 192.168.1.102:443>
    ServerName api.example.com
</VirtualHost>

Database Replication¶

network:
  version: 2
  ethernets:
    eth0:
      addresses:
        - 192.168.1.100/24    # Client connections
        - 10.0.0.100/24       # Replication traffic

Loopback Addresses¶

Additional Loopback IPs¶

network:
  version: 2
  ethernets:
    lo:
      addresses:
        - 127.0.0.1/8         # Standard
        - 10.255.255.1/32     # Service IP
        - 10.255.255.2/32     # Another service

Loopback IPs are useful for: - BGP anycast - Service addresses - Testing

Dynamic Address Management¶

Adding Addresses Dynamically¶

While netplan manages static config, you can add temporary addresses:

# Add temporary address
ip addr add 192.168.1.150/24 dev eth0

# Remove temporary address
ip addr del 192.168.1.150/24 dev eth0

Persistence

Manually added addresses are lost on reboot or netplan apply.

Address Order¶

Primary Address Selection¶

The first address is typically used as the source for outgoing connections:

addresses:
  - 192.168.1.100/24    # Used as source IP
  - 192.168.1.101/24    # Secondary

Explicit Source Selection¶

Some applications allow specifying source IP:

# Ping from specific source
ping -I 192.168.1.101 8.8.8.8

# curl with source
curl --interface 192.168.1.101 https://example.com

Verifying Configuration¶

Check All Addresses¶

# All addresses
ip addr show

# Specific interface
ip addr show dev eth0

# Just IPv4
ip -4 addr show dev eth0

# Just IPv6
ip -6 addr show dev eth0

Check Address Labels¶

ip addr show dev eth0 | grep "inet.*label"

Test Connectivity from Each Address¶

# Test each IP
for ip in 192.168.1.100 192.168.1.101 192.168.1.102; do
    echo "Testing $ip"
    ping -c 1 -I $ip 8.8.8.8
done

Troubleshooting¶

Address Not Added¶

# Check netplan syntax
sudo netplan generate

# Check systemd-networkd logs
journalctl -u systemd-networkd -f

# Verify generated config
cat /run/systemd/network/*netplan*.network

Duplicate Address¶

# Check for duplicates
ip addr show | grep "192.168.1.100"

# May appear on multiple interfaces after misconfiguration

Wrong Source Address¶

# Check routing table for source
ip route get 8.8.8.8

# May need policy routing for specific source requirements

Best Practices¶

Document all addresses - Keep a record of what each IP is for
Use labels - Makes troubleshooting easier
Plan CIDR carefully - Avoid redundant routes
Consider security - Each IP may need firewall rules
Test bindings - Verify services bind to correct IPs
Use DNS - Don't rely on remembering IPs